C-arm units are commonly used in order to obtain X-ray images of bodily parts during surgery. C-arm trackers have been integrated in computer-assisted surgery (CAS), by the addition of a C-arm tracker to a C-arm unit, in order to allow image-based navigation. As the accuracy of the computer-assisted surgery is dependent on the quality of the images, the C-arm tracker is used to relate the X-ray image to the tracking of the bodily part.
The relation between the X-ray image and the tracking of the bodily part is obtained by tracking the C-arm tracker to provide a scale relation between the image and the bodily part. Because of accuracy requirements for CAS surgery, the positional relation between the C-arm tracker and the tracked bodily part must be recorded at the time that the image is taken, for instance to avoid errors due to movement between the patient and the C-arm tracker, from the moment the X-ray is taken and to the moment the X-ray image file is converted to navigation data.
Accordingly, diodes are commonly used to detect X-rays. More specifically, diodes are provided on the C-arm tracker and are wired to the CAS processor system so as to detect and signal the detection of X-rays. With the signaling of the X-ray, the CAS processor system simultaneously records the positional relation between the C-arm tracker and the patient. The positional relation is then used in the conversion of the X-ray image to navigation data, for instance to adjust the orientation of the image.